The present invention relates generally to apparatus for optically recording and/or reproducing information on and from an optical disk and, more particularly, the present invention relates to a device for feedback controlling focus of an optical system of such an optical recording and/or reproducing system so that a light spot on a record disk is focused irrespective of the disk attitude.
Various types of information recording and/or reproducing systems for deriving prerecorded information on a disk are known. In one system, the prerecorded information is reproduced optically by irradiating an optical disk surface with a light beam. It is desirable for such a light beam to be focused so it is incident at a predetermined point on the disk, to provide a minute spot. However, the focusing condition is apt to be interrupted because of variations in distance between a condenser lens of an optical system and an information track of the rotating optical disk. Conventional systems compensate for such defocusing caused by these distance variations with an automatic feedback control system that maintains the distance between a condenser lens of an optical system and an illuminated information track in the optical system constant by driving the condenser lens in a direction normal to the disk. Such a conventional feedback control system, referred to as a focus controlling system, produces a focusing error signal to indicate the degree of defocusing. Two prior art systems for producing such a focusing system are known. In one prior system, an astigmatic system uses a condenser lens, a cylindrical lens and a four-quadrant composite optical sensor. In a second prior art knife-edge system, a condenser lens, a knife-edge and two-section composite optical sensor are employed. The prior art systems, however, require a relatively long optical path because it is necessary to reimage a light spot focused on the disk at some place in the optical path. Such a long optical path results in a bulky, and difficult to use optical system.
Another conventional feedback focus controlling system uses a detecting prism that is positioned so that a reflecting surface thereof makes a critical angle with respect to an optical axis. Light reflected from the reflecting surface is applied to a two-section composite optical sensor. To increase the sensitivity of such a system, it is necessary to use a prism having a reflecting surface which reflects a light beam several times. Such a prism is bulky, causing the entire optical system to become large. In addition, the size of the optical system is relatively large because of the requirement for refraction or off-set of the optical system. Furthermore, this system requires difficult, time consuming adjustment during manufacturing, resulting in high manufacturing costs.